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Provide short answers to the following questions:

Disk requests come to disk driver for cylinders $10, 22, 20, 2, 40, 6$ and $38$, in that order at a time when the disk drive is reading from cylinder $20$. The seek time is $6$ msec per cylinder. Compute the total seek time if the disk arm scheduling algorithm is.

  1. First come first served.
  2. Closest cylinder next.
in Operating System by Boss (30.8k points) | 1.2k views

2 Answers

+22 votes
Best answer
  1. In FCFS sequence will be $\Rightarrow 20, 10, 22, 20, 2, 40, 6, 38$
         total movement: $ |20-10| + |10-22| + |22-20| + |20-2| + |2-40| + |40-6| + |6-38| = 146$
        so total seek time $= 146 \times 6 = \mathbf{876 msec}$


     
  2. In Closest cylinder next sequence will be $\Rightarrow 20, 22, 10, 6, 2, 38, 40$
         total movement: $|20-22| + |22-2| + |2-40| = 60$
         so total seek time $= 60 \times 6 = \mathbf{360 msec}$
by Boss (11.5k points)
edited by
0
For FCFS

In last one |6-40| ?
0
sry thats typo..its |6-38|
0

admin please correct the part B of this answer. same error is in book  !!!

0
What error?
0

@Arjun Sir

For better clarity it should be 

Total head movements $ = \:\mid 20-22\mid + \mid 22-10\mid + \mid 10-6 \mid + \mid 6 - 2 \mid + \mid 2-38 \mid + \mid 38-40 \mid$

$ = 2 + 12 + 4 + 4 + 36 + 2 = 60$

So, total seek time $ = 60 \times 6\:msec = 360\:msec$

+11 votes

A)FCFS:(20-10) + (22-10) + (22-2) + (40-2) + (40-6) + (38-6) = 146

But seek time is 6ms per cylinder,So total seek time = 146 * 6 = 876.

B)Closest cylinder next:(22-20) + (22-10) + (10-6) + (6-2)  + (38-2) + (40-38) = 60

So total seek time = 60 * 6 = 360.

by Boss (12.5k points)
edited by
0

in FCFS 20 should be visited again after 22

in closest cylinder next 22 should be visited from 20

0
20 is not visited again because it is initially at 20.

closest cylinder next ..corrected.
+7
in FCFS it doesn't matter its initial position...he follows the sequence without knowing whether he will visit initial position again
so as per the sequence 20 will be visited again
0
AFAIK,your statement is  not correct.
0

Which statement @Prabhanjan_1?

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